The present invention relates to a camera module. More specifically, the present invention relates to a camera module having a lens chip and a spacer portion for bonding the lens chip. The present invention also relates to a method of producing the camera module.
Recently, a demand for a small camera module has been increasing in an application such as a mobile phone camera module and a security monitoring camera module. Among components constituting the camera module, an image element for converting an image to an electrical signal tends to be a most expensive component. The image element is generally formed of a CMOS (Complementary Metal Oxide Semiconductor) image sensor.
The CMOS image sensor can be manufactured through an ordinary semiconductor manufacturing process. Accordingly, when a size of the CMOS image sensor decreases, it is possible to drastically reduce a cost of the CMOS image sensor. For this reason, a total size of the camera module and a size of an optical lens disposed in the camera module have been decreasing rapidly. Patent Reference has disclosed a technology for further reducing a cost of the camera module, in which the camera module is assembled in a wafer level, thereby producing the camera module of a chip size package (CSP) type.    Patent Reference: Japanese Patent Publication No. 2004-063751
According to the technology disclosed in Patent Reference, first, a glass substrate and a semiconductor substrate are prepared. The glass substrate is provided with a plurality of lenses integrally formed thereon. The semiconductor substrate is provided with a plurality of sensors integrally formed thereon. Then, the glass substrate is positioned and aligned with the semiconductor substrate in a wafer level, so that each of the lenses is aligned with each of the sensors, thereby collectively aligning optical axes thereof. After the glass substrate is bonded to the semiconductor substrate, the glass substrate and the semiconductor substrate thus bonded are diced (individually separated), thereby obtaining the camera module.
When it is required to reduce a thickness of a mobile phone, it is necessary to reduce a size or a height of the camera module to be mounted thereon accordingly. As a result, it is necessary to reduce a thickness of the semiconductor substrate or the sensor wafer.
In general, when it is necessary to reduce a thickness of the sensor wafer, a back-grind method is adopted. In the back-grind method, a wafer holding surface as one side surface of the sensor wafer is held in a wafer holder, and a wafer grinding surface as the other side surface of the sensor wafer is pressed against a polisher stage plate, so that the sensor wafer is ground while a polishing agent is being dropped. When the sensor wafer is ground using the polishing agent, an alkali type cleaning liquid is used for removing the polishing agent.
In order to reduce a thickness of the sensor wafer, there are generally two steps, i.e., a grinding process in which a backside surface of the sensor wafer is ground with a grind stone, and a CMP (chemical Mechanical Polishing) process in which the backside surface is further polished to obtain a mirror surface.
When the backside surface of the sensor wafer is ground with the grind stone, it is necessary to grind the backside surface uniformly. Accordingly, in order to improve a manufacture yield of a semiconductor chip after the sensor wafer is diced, it is imperative to securely hold the sensor wafer with the wafer holder. Further, when a ground dust or a polishing agent contacts with the wafer holding surface with an electric circuit such as a sensor formed thereon, the sensor wafer may be damaged. Accordingly, it is necessary to securely protect the wafer holding surface during the grinding process. Still further, when the sensor wafer is ground, the sensor wafer tends to warp. Accordingly, it is necessary to correct the warpage of the sensor wafer during the grinding process.
To this end, when the backside surface of the sensor wafer is ground with the grind stone, a back-grind adhesive sheet and a jig formed of stainless steel called a guide ring are used. The guide ring has an opening at a center portion thereof having a diameter greater than that of the sensor wafer. The guide ring is attached to the adhesive sheet, and the sensor wafer is attached to the adhesive sheet at the opening. In this step, the wafer holding surface with the sensor formed thereon is attached to the adhesive sheet, thereby fixing the sensor wafer and protecting a foreign matter from attaching to the wafer holding surface. Accordingly, it is possible to securely protect the sensor wafer during the grinding process.
When the sensor wafer is attached to the adhesive sheet with a small adhesion strength, the sensor wafer may move or be peeled off from the adhesive sheet due to the warpage thereof. On the other hand, when the sensor wafer is attached to the adhesive sheet with an excessive adhesion strength, it is difficult to remove the sensor wafer from the adhesive sheet or damage the wafer upon removing from the adhesive sheet after the grinding process. Accordingly, it is necessary to attach the sensor wafer to the adhesive sheet with an optimal adhesion strength.
After the sensor wafer is attached to the adhesive sheet, the guide ring is installed in a grinding device. In the grinding device, the guide ring rotates together with the grinding stage while the sensor wafer is pressed against the grinding stage with an appropriate force, thereby grinding the backside surface of the sensor wafer. In the grinding process, the sensor wafer is attached to the adhesive sheet with an appropriate adhesion force to prevent the warpage thereof. Accordingly, it is possible to prevent the sensor wafer from warping even when the thickness of the sensor wafer decreases.
After the sensor wafer is ground to have a specific target thickness, the backside surface of the sensor wafer is polished to obtain the mirror surface. In the polishing process, a polishing cloth and a polishing liquid are used. More specifically, the guide ring rotates together with the polishing cloth while the sensor wafer is pressed against the polishing cloth with an appropriate force, thereby polishing the backside surface of the sensor wafer.
Patent Reference does not disclose a method of grinding a backside surface of the semiconductor wafer in detail. In the manufacturing process of the CSP camera module, the wafer holding surface of the sensor wafer (the surface with the sensor formed thereon) is generally covered with a cover glass before the grinding process, thereby preventing the sensor from being contaminated or damaged during the grinding process or the dicing process of the sensor wafer.
In Patent Reference, the glass substrate as a transparent substrate is provided with a convex lens on an outer side thereof. The glass substrate as the cover glass covers the semiconductor wafer with the sensor, and the backside surface of the semiconductor wafer is ground. In this case, only a convex portion of the convex lens is attached to the adhesive sheet. Accordingly, it is difficult to attach the glass substrate to the adhesive sheet with a sufficient adhesion strength, thereby causing unstable adhesion in the grinding process.
In view of the problems described above, an object of the present invention is to provide a camera module and a method of producing the camera module capable of solving the problems of the conventional technology. In the present invention, it is possible to reduce the number of manufacturing steps and improve a manufacturing yield.
Further objects and advantages of the invention will be apparent from the following description of the invention.